The present invention relates to a crosslinking process especially in the field of (meth)acrylic resins.
The process according to the invention is used for the preparation of molded, impregnated, coated and printed articles, adhesives, binders, finishes, paints and varnishes.
In the latter applications, since the plastic materials are associated with metallic articles, especially in the case of automobile bodies, it is extremely desirable to have coating compositions available which crosslink at low temperatures and substantially at temperatures lower than the transition temperatures of these plastics.
Hitherto, all kinds of polymers and polycondensates in the form of solutions, dispersions or powders have been used in the field of thermosetting paints and coatings. By way of example, the alkyd resins, polyesters, acrylic resins, epoxide resins obtained, for example, by condensation of derivatives of bis-phenol or polyoxyalkylene glycol with epichlorohydrin, polyurethanes prepared from polyols and isocyanates, and polyamides and the like may be mentioned.
Amongst these, the epoxide resins and especially epoxides of aromatics are known to be very sensitive to the action of light and weathering. The resins utilizing isocyanates, which are admittedly very toxic, raise application problems in turn.
Amongst the others, there has been particular interest in acrylic resins in the form of a dispersion and more generally in solution in organic solvents. When applied at high temperatures (generally between 130.degree. C. and 200.degree. C. in the course of approximately 30 minutes), they make it possible to obtain coatings provided not only with good mechanical characteristics but likewise good properties with respect to durability and resistance to weathering, water, solvents and chemical agents in general.
Unfortunately, the coatings which are thus obtained and are crosslinked at low temperatures, for example at 90.degree. C. for 30 minutes, are relatively sensitive to prolonged action of water and are not suitable for the applications mentioned above.
To mitigate this disadvantage, acrylic copolymers prepared from glycidyl acrylate or methacrylate have been claimed. These copolymers are crosslinked by mixing with "crosslinking agents" such as: the dicarboxylic or polycarboxylic acids mentioned in U.S. Pat. No. 2,857,354 or French Patent 2,149,409, the dicarboxylic acid anhydrides quoted in Belgian Patent 819,623, the phenol/formaldehyde resins, polyamide resins or urea/formaldehyde resins indicated in U.S. Pat. No. 3,058,947 or the resins containing amine groups, amide groups, acid groups or thiol groups, as mentioned in European Patent 0,129,394. Generally, these crosslinkings are promoted by the presence of known catalysts in this mixture, such as tetraalkylammonium salts, imidazole derivatives, tertiary amines or carboxylic acid salts. These compositions, applied in thin layers to metallic substrates, lead, after heat treatment at a low temperature, to coatings, the water resistance of which is in fact relatively good. Unfortunately, since crosslinking is incomplete under these conditions, the solvent resistance in inadequate. Moreover, the compositions prepared with the aid of the above mixtures above all undergo changes in time and end up by setting fairly rapidly to a solid mass in the pot.
To improve the reactivity of these systems, copolymers of glycidyl acrylate or methacrylate have been provided, in which these monomers are copolymerized with functional monomers carrying hydroxyl, carboxyl, anhydride, amine or amide functional groups, in order to make them "self-crosslinking", that is to say crosslinkable in the absence of crosslinking agents.
In this case, since the crosslinking can then take place in the absence of the "crosslinking agents" indicated above, it is possible to prepare simplified coating compositions by mixing these copolymers with the known catalysts, and thus without crosslinking agents. These compositions lead to coatings which have good water and solvent resistance and in addition show good general properties. Unfortunately, the stability of the composition is limited to a few hours, after which it sets to a solid mass in the pot.